The present invention relates to an ultraviolet-curable organopolysiloxane composition or, more particularly, to an organopolysiloxane composition capable of being cured by exposure to ultraviolet light within a relatively short time to give a cured rubbery elastomer having excellent water-resistance and useful as an adhesive, sealing material, FIPG material, potting agent, coating agent and the like.
In the prior art, several types of ultraviolet-curable organopolysiloxane compositions are known, of which those disclosed in Japanese patent publication 54-6512 are prepared by compounding a first organopolysiloxane having vinyl groups directly bonded to the silicon atoms, a second organopolysiloxane having mercaptoalkyl groups bonded to the silicon atoms and an initiator for the photochemical reaction between the vinyl groups and the mercapto groups. A disadvantage in the ultraviolet-curable organopolysiloxane composition of this type is that the photochemical reaction is susceptible to inhibition by oxygen so that curing of the composition is sometimes incomplete in the portion in contact with air and complete curing can be obtained only by exposure to ultraviolet light in an atmosphere of an inert gas.
With an object to overcome the above mentioned disadvantage in organopolysiloxane composition curable by the addition reaction between silicon-bonded vinyl groups and mercapto groups, an improved organopolysiloxane composition has been previously proposed which comprises a first organopolysiloxane having silanolic hydroxy groups, a second organopolysiloxane having silicon-bonded isopropenyloxy groups, a third organopolysiloxane having Mercapto groups, a curing agent and a photosensitizer. The composition of this type is curable by two different mechanisms, one being an ultraviolet-induced addition reaction and the other being a condensation reaction which proceeds in the presence of moisture so that the composition is advantageous with respect to the possibility of complete curing even in the portion not under direct exposure to ultraviolet or when the composition contains a coloring agent. A problem in the composition of this type is its relatively poor adhesive bonding strength immediately after exposure to ultraviolet light.
It has been discovered that the ultraviolet-curable organopolysiloxane composition of the above described type can be imparted with an improved incipient adhesive bonding strength immediately after exposure to ultraviolet light when the composition is admixed with an organosilane compound or organopolysiloxane compound having at least three hydrolyzable groups in the molecule. The admixture of such an additive, however, causes some problems, viz., the composition is relatively unstable and sometimes becomes cloudy when kept under an atmosphere of high humidity and the cured rubber obtained from the composition has a relatively poor mechanical strength.